Stents are ridged, or semi-ridged, tubular scaffoldings that are deployed within the lumen (inner tubular space) of a vessel or duct during angioplasty or related procedures intended to restore patency (openness) to vessel or duct lumens. Stents generally are left within the lumen of a vessel or duct after angioplasty or a related procedure to reduce the risk of restenosis (re-closure).
During angioplasty, an expandable stent typically is mounted over a balloon catheter and positioned at a desired location within the lumen of a vessel or duct. The balloon is inflated temporarily thereby expanding and implanting the stent within the lumen. The balloon then is deflated and removed from the lumen, while the stent is left in place. Because stent and balloon implantation is a relatively non-invasive procedure, it has proven to be a preferable alternative to open heart surgery.
Due to their generally straight and tubular shape, conventional stents can be effective at restoring patency to the lumens of vessels or ducts when the area to be treated is a uniform and relatively straight area of the vessel or duct. Vessels and ducts, however, branch numerous times as they travel throughout the body. When a vessel branches, the opening to the branched vessel is called an ostium. Conventional stents are not adequate to restore patency around ostiums for several reasons. First, in many circumstances, patency must be restored in a main vessel both before and after an ostium at a vessel bifurcation. A conventional stent cannot restore patency both before and after an ostium without covering the ostium itself, thereby “gating” it. Such “gating” generally is not acceptable because it impedes blood flow to the vessel branch. If the ostium is not gated in this manner, however, then patency has not been restored both before and after the ostium with the use of a conventional stent. While two conventional stents can be used to restore patency around an ostium (one before and one after the ostium), this approach leaves the ostium area itself untreated. Second, if the area to be treated extends into the branched portion of the vessel, the conventional stent does not restore patency to the branch because it remains entirely within the main artery. Thus, treatment is incomplete.
One type of stent that has been developed to address the problem of restoring patency near vessel bifurcations is the distally-flaring stent. Distally-flaring stents, as described in U.S. Pat. No. 5,868,777 (the “Lam patent” issued Feb. 9, 1999 and assigned to Advanced Cardiovascular Systems, Santa Clara, Calif.), have one end that is highly malleable and able to conform to the irregular shape of an ostium. When placed on a balloon catheter, and during deployment, the highly-malleable (“flaring” or “flanged” portion) of the stent is positioned on the distal (far) end of the balloon catheter. The distally-flaring stent is advanced in a retrograde fashion (i.e., against the direction of blood flow or towards the branch's main artery) until its distal end is in the immediate vicinity of the ostium of a side branch. The main body of the distally-flaring stent then is expanded within the side branch, while the distally-flaring portion is expanded over the ostium of the branch and, to a small degree, within the main artery. In this manner, the distally-flaring stent may be adequate to restore patency when a lesion is located primarily within a side branch and at an ostium (see FIG. 1) and when the treatment site can be approached from an entry point that is distal to the ostial narrowing (for example the superior or inferior extremity arteries).
While the above described method of using distally-flaring stents can be effective at restoring patency at particular branches, there are many circumstances when the treatment site cannot be approached from a point distal to the ostial narrowing (for example, in the coronary arteries). In these circumstances, the distally-flaring stent is inadequate because if it is advanced in a non-retrograde fashion (i.e., into the side branch from the main artery), and it is positioned within a side branch to restore patency, the distal flaring portion of the stent will have moved beyond the ostium and is no longer in position to expand in the area that it was designed to treat. Instead, once positioned within a side branch, the distally-flaring portion of the stent is confined within the more uniform lumen of the vessel, removed a distance from the irregular shape of the ostium. Thus, in an area of restricted access, such as the coronary arteries (and, as will be explained infra, other lumens as well), a distally-flaring stent is not effective in treating a lesion found within a side branch and ostium. Therefore, a need exists for a stent that is able to expand predominantly within a side branch while still flaring to cover the irregular shape of an ostium when the ostium only can be approached from the direction of the main artery.
Importantly, one may believe that the drawbacks associated with the distally-flaring stent of the Lam patent may be overcome simply by turning the distally-flaring stent around prior to loading it onto a balloon of a balloon catheter. This, however is not the case. Simply turning the distally-flaring stent around does not address the drawbacks associated with distally-flaring stents because this ‘solution’ does not provide a way to ensure that the flaring portion of the stent will be fully deployed against the interior walls of a main artery. Indeed, when loaded onto a balloon, the distally-flaring stents described in the Lam patent cover substantially the balloon's entire length. This extensive coverage of the balloon by the distally-flaring stent prevents the balloon from inflating meaningfully beyond the stent's proximal end which can prevent the balloon from pushing the proximal end of the stent entirely flush against the ostium thus occluding blood flow in the main artery. The Lam patent does not provide an adequate solution to this problem.
One approach to address the failure to inflate meaningfully beyond the stent's proximal end is to pull the balloon catheter back relative to the position of the stent and inflate the balloon again. Such repositioning of the balloon catheter, however, generally will negatively affect the positioning of the stent. Thus, this approach also does not provide an adequate solution to the above-identified problems.
Based on the preceding discussion, the distally-flaring stents described in the Lam patent can only be used effectively in a manner that is retrograde to the direction of blood flow (for example at ostiums of superior and inferior extremity arteries). The distally-flaring stents described in the Lam patent may be effective in these areas because they may have an entry point that is distal to the ostium. The distally-flaring stents described in the Lam patent cannot, however, be used to treat side branches and ostiums in the coronary arteries or other branches that may only be approached in the direction of blood flow.